1. Field of the Invention
The subject invention relates to a method of separating dialkyl maleate from monoalkyl maleate to produce essentially pure dialkyl maleate while minimizing reversion of monoalkyl maleate to maleic anhydride and alkanol. The invention also relates to a method of producing substantially pure dialkyl maleate free of maleic anhydride, monoalkyl maleate, and alkanol from maleic anhydride by advantageously utilizing this separation method.
2. Description of the Related Art
Production of 1,4-butanediol, gamma-butyrolactone, and tetrahydrofuran from maleic anhydride is typically carried out in stages. According to a known procedure, maleic anhydride first is esterified with ethanol to produce monoethyl maleate. This reaction proceeds rapidly without catalyst. The monoethyl maleate then is esterified with ethanol to produce diethyl maleate. The second esterification can be catalyzed if desired. Water is a by-product. The desired 1,4-butanediol, gamma-butyrolactone, and tetrahydrofuran products are obtained from diethyl maleate by hydrogenolysis, using, for example, a catalyst such as that disclosed in Sharif, U.S. Pat. No. 4,584,419.
To maximize the life of the hydrogenation catalyst, it is desirable that the dialkyl maleate be essentially free of monoalkyl maleate. One technique of obtaining dialkyl maleate free of monoalkyl maleate is to obtain essentially complete conversion in the esterification stages of the process.
The monoethyl maleate to diethyl maleate reaction is typically catalyzed in a first reactor, such as a stirred tank or plug flow reactor, until the conversion level is about 70 percent. Because the reaction is an equilibrium reaction, the water present precludes obtaining higher conversion levels in a single reactor. Instead, to obtain complete conversion of monoethyl maleate to diethyl maleate, a water/ethanol mixture is removed and dry ethanol is fed to additional reactors. The equilibrium nature of the reaction requires that an essentially dry reaction system be maintained in the reactor in which complete conversion is obtained.
The requirement that water be removed from the reaction system to shift the equilibrium and allow the reaction to go to completion imposes a substantial burden on such an operation. For example, removal of water from the mixture also entails the removal of substantial quantities of ethanol, because these components form an azeotrope. To obtain dry ethanol for the process, the water must be separated in an energy-intensive and costly separation.
The alternative to completion of the reaction, neutralizing the mono-esterified component of the incompletely-esterified stream with base, is also unsatisfactory. Not only is reactant wasted, but additional costs are incurred in producing a diethyl maleate stream free of monoethyl maleate. A costly waste-treatment facility will be required. The monoethyl maleate which is neutralized is lost, e.g. by delivery to a waste treatment facility unless the base stream is acidified and the monoethyl maleate extracted, adding yet additional cost to the system. Also, the consumption of base imposes an additional cost on the process.
High purity diester product is necessary because the diethyl maleate is utilized, inter alia, as a reactant in the production of 1,4-butanediol by catalytic hydrogenolysis, and the catalyst commonly used for this reaction typically is deactivated by carboxylic acids such as monoethyl maleate.
In a process requiring complete conversion of monoethyl maleate to the diester, the expense associated with completely dehydrating ethanol and of maintaining a dry reaction system is significant. The alternative approach, wherein monoethyl maleate is neutralized, is technically and commercially feasible only when monoethyl maleate is present at low concentrations. Therefore, a method of physically separating monoethyl maleate from diethyl maleate would be desirable. However, such a method heretofore has been unavailable.
Attempts to separate monoethyl maleate from diethyl maleate by distillation have been unsuccessful because monoethyl maleate reverts to maleic anhydride and ethanol at conditions typically required to effect the separation, i.e., when using conventional residence time, high temperature distillations. Monoethyl maleate exposed to these conditions reverts to ethanol and maleic anhydride at the bottom of the distillation tower. At the top of the tower, ethanol and maleic anhydride recombine to yield monoethyl maleate. Therefore, a substantial internal flow develops within the distillation column, causing severe operational difficulty. Further, monoethyl maleate is removed from the tower in the tails, which contain primarily diethyl maleate, thus defeating the purpose of the separation, viz, to produce pure diethyl maleate free of monoethyl maleate. Thus, production of pure diethyl maleate is impossible if this technique alone is used.
It is an object of this invention to substantially completely separate dialkyl maleate from monoalkyl maleate, such as diethyl maleate from monoethyl maleate, by distillation while minimizing reversion of the monoalkyl maleate to maleic anhydride and alkanol (ethanol), and without having to neutralize the monoalkyl maleate.
It is a further object of this invention to produce substantially pure diethyl maleate without having to use anhydrous ethanol to convert monoethyl maleate to diethyl maleate.